Math.easeInOutQuad = function(t, b, c, d){
	t /= d / 2;
	if(t < 1){
		return (c / 2) * t * t + b;
	}
	t--;
	return (-c / 2) * (t * (t - 2) - 1) + b;
};

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function(){
	return (
		window.requestAnimationFrame
		|| window.webkitRequestAnimationFrame
		|| window.mozRequestAnimationFrame
		|| function(callback){
    	window.setTimeout(callback, 1000 / 60);
		}
	);
})();

/**
 * Because it's so fucking difficult to detect the scrolling element, just move them all
 * @param {number} amount
 */
function move(amount){
	document.documentElement.scrollTop = amount;
	document.body.parentNode.scrollTop = amount;
	document.body.scrollTop = amount;
}

function position(){
	return (
		document.documentElement.scrollTop
		|| document.body.parentNode.scrollTop
		|| document.body.scrollTop
	);
}

/**
 * @param {number} to
 * @param {number} duration
 * @param {Function} callback
 */
export function scrollTo(to, duration, callback){
	const start = position();
	const change = to - start;
	const increment = 20;
	let currentTime = 0;
	duration = typeof duration === 'undefined' ? 500 : duration;
	const animateScroll = function(){
    // increment the time
		currentTime += increment;
    // find the value with the quadratic in-out easing function
		const val = Math.easeInOutQuad(currentTime, start, change, duration);
    // move the document.body
		move(val);
    // do the animation unless its over
		if(currentTime < duration){
			requestAnimFrame(animateScroll);
		}else{
			if(callback && typeof callback === 'function'){
        // the animation is done so lets callback
				callback();
			}
		}
	};
	animateScroll();
}